A turbofan gas turbine engine comprises a fan, which comprises a fan rotor and a number of circumferentially spaced radially outwardly extending fan blades secured to the fan rotor. The fan is surrounded by a fan casing, which defines a fan duct. The fan casing is arranged to contain one or more of the fan blades in the unlikely event that a fan blade becomes detached from the fan rotor. Safety regulations dictate that the worst case of this scenario—in which the whole blade is released—must be provided for in the design of the fan casing.
If a fan blade becomes detached from the fan rotor, the released fan blade will strike a main fan casing containment region. The fan blade will generally break up progressively under a buckling action. The fan blade increases in strength from the tip to the root and at some position between the tip and the root the remaining portion of the fan blade, including the root, will no longer buckle. The remaining portion of the fan blade has substantial mass and will be accelerated to impact a rear fan containment region of the fan casing.
It is necessary to provide additional material to the rear fan containment region of the fan casing to contain the remaining portion of the fan blade. The additional material may be in the form of an increase in thickness, the provision of ribs, honeycomb liners etc, which dissipate the impact energy by plastic deformation of the material. However, these methods of protecting the rear fan containment region add weight to the turbofan gas turbine engine.
It is known to provide apertures or passages in the root to promote break up of the root upon impact such that there is a reduced load placed upon the rearward portions of the fan casing. This permits a reduction in the reinforcement required in the casing and a reduction of weight.
In the case of fan blades formed of metal, such apertures or passages can be formed by drilling. In the case of blades formed of composite materials, drilling can provide a moisture path into the composite, which can lead to laminar cracking.